Non-Volatile RAM (NVRAM) is a very useful device, because the data recorded in it will not disappear even without power. At present, the [only available] most promising NVRAM is the ferroelectric one. Other nonvolatile memory devices are not truly randomly accessible for writing, reading and rewriting. In principle, the ferroelectric NVRAM is essentially a DRAM whose capacitors use ferroelectric material as insulator. The data recording unit cell of most conventional DRAM consists of an MOSFET and a capacitor. Data are recorded as charge stored in the capacitor while the MOSFET serves as an address selector. Because of discharge through inevitable leakage, charge stored in the capacitor will disappear after a certain time. So, DRAM is a volatile memory device. However, if we replace the capacitor in the unit cell of the DRAM by a special capacitor with ferro-electric film as insulator, we will get a nonvolatile RAM. In this case, data are recorded not as charge in the capacitor, but as polarization in the ferroelectric layer. Depending on the direction of the electric field applied across it, the ferroelectric layer can have two direction of polarization. We can use one direction to represent the digit "1", the other direction "0". For ferroelectric material, the polarization will not disappear when we turn off the applied field. This is the foundation of the data nonvolatility of ferroelectric RAM.
Nowadays ferroelectric NVRAM has a fatigue problem that hinders its use. The remnant polarization of the ferroelectric material will diminish after many reversals of the direction of polarization, leading ultimately to practically indistinguishability of two states of data record. Research showed this problem arises from the oxygen vacancies or other defects within the ferroelectric material, such as PZT. A high temperature annealing can eliminate these defects and alleviate the fatigue problem. However, if the ferroelectric capacitors and the MOSFETs are fabricated on a same substrate, annealing at higher than 550 degree centigrade will have adverse effects (such as unwanted diffusion of the contact metal into the silicon and excessive diffusion of the dopants) on the MOS structure. (See the article "Progress in Ferroelectric Memory Technology," by William A. Geideman of McDonnell Douglas Electronic Systems Company, IEEE Trans. on Ultrasonic, Ferroelectrics and frequency Control, 38, p. 704-711, (1991).) Unfortunately, annealing temperature higher than 600.degree. C. are desired for annealing PZT.
If we fabricate the ferroelectric capacitors on another substrate, not on the substrate with MOSFETs, then we can anneal the ferroelectric material at temperature higher than 600.degree. C., without any adverse effect on the MOS structure. This is one advantage of fabricating the NVRAM in two parts.
Many other ferro-electric materials are not compatible with the MOS technology. If the ferroelectric capacitors are fabricated on another substrate, then we can avoid such incompatibility and have a wider choice of ferroelectric materials. This is another advantage of fabricating the device in two parts.
In summary, fabricating the whole NVRAM in two parts facilitates its manufacture, provides more choices of materials and fabrication methods. Moreover, the total yield may be higher because either part is made with fewer steps and in simpler construction. The data recording part is especially simple in construction. It can be manufactured in large quantity at low cost.
Since the invention of IC, more and more devices are being put into a single chip. Because different devices require different processing procedures, the whole processing procedure is getting more and more complex. The yield is getting lower and lower and the cost higher and higher. Fabricating a complex IC in two parts is a new way that breaks away from the above trend and will lead to a new area of IC manufacturing.
Of course, after fabricating the whole device in two parts, we should make the required electrical connections between them. We will explain how to solve this key issue later on.